The mktemp() family of functions take the given file
name template and overwrite a portion of it to create a new file name. This
file name is unique and suitable for use by the application. The template may
be any file name with at least six trailing Xs, for example
/tmp/temp.XXXXXXXX. The trailing Xs
are replaced with a unique digit and letter combination. The number of unique
file names that can be returned depends on the number of Xs
provided; mktemp() will try at least 2 ** 31
combinations before giving up. At least six Xs must be used,
though 10 is much better.

The mktemp() function generates a
temporary file name based on a template as described above. Because
mktemp() does not actually create the temporary file
there is a window of opportunity during which another process can open the
file instead. Because of this race condition,
mktemp() should not be used where
mkstemp() can be used instead.
mktemp() was marked as a legacy interface in
IEEE Std 1003.1-2001 (“POSIX.1”).

The mkstemp() function makes the same
replacement to the template and creates the template file, mode 0600,
returning a file descriptor opened for reading and writing. This avoids the
race between testing for a file's existence and opening it for use.

The mkostemp() function acts the same as
mkstemp(), except that the
flags argument may contain zero or more of the
following flags for the underlying open(2)
system call:

The mkstemps() and
mkostemps() functions act the same as
mkstemp() and mkostemp(),
except they permit a suffix to exist in the template. The template should be
of the form /tmp/tmpXXXXXXXXXXsuffix.
mkstemps() and mkostemps()
are told the length of the suffix string, i.e.,
strlen("suffix").

The mkdtemp() function makes the same
replacement to the template as in mktemp() and
creates the template directory, mode 0700.

The mktemp() and mkdtemp()
functions return a pointer to the template on success and
NULL on failure. The
mkstemp(), mkostemp(),
mkstemps(), and mkostemps()
functions return -1 if no suitable file could be created. If any call fails,
an error code is placed in the global variable errno.

Quite often a programmer will want to replace a use of
mktemp() with mkstemp(),
usually to avoid the problems described above. Doing this correctly requires a
good understanding of the code in question.

Often one will find code which uses
mktemp() very early on, perhaps to globally
initialize the template nicely, but the code which calls
open(2) or
fopen(3) on that file name will occur much
later. (In almost all cases, the use of
fopen(3) will mean that the flags
O_CREAT | O_EXCL are not
given to open(2), and thus a symbolic link
race becomes possible, hence making necessary the use of
fdopen(3) as seen above.) Furthermore,
one must be careful about code which opens, closes, and then re-opens the
file in question. Finally, one must ensure that upon error the temporary
file is removed correctly.

There are also cases where modifying the code to use
mktemp(), in concert with
open(2) using the flags
O_CREAT | O_EXCL, is better,
as long as the code retries a new template if
open(2) fails with an
errno of EEXIST.

The mkdtemp() and mkstemp()
functions conform to the IEEE Std 1003.1-2008
(“POSIX.1”) specification. The ability to specify more
than six Xs is an extension to that standard. The
mkostemp() function is expected to conform to a future
revision of that standard.

The mktemp() function conforms to
IEEE Std 1003.1-2001 (“POSIX.1”); as
of IEEE Std 1003.1-2008 (“POSIX.1”) it
is no longer a part of the standard.

The mkstemps() and
mkostemps() functions are non-standard and should
not be used if portability is required.

A mktemp() function appeared in
Version 7 AT&T UNIX. The
mkdtemp() function appeared in
OpenBSD 2.2. The mkstemp()
function appeared in 4.4BSD. The
mkstemps() function appeared in
OpenBSD 2.3. The mkostemp()
and mkostemps() functions appeared in
OpenBSD 5.7.

For mktemp() there is an obvious race between file name
selection and file creation and deletion: the program is typically written to
call tmpnam(3),
tempnam(3), or
mktemp(). Subsequently, the program calls
open(2) or
fopen(3) and erroneously opens a file (or
symbolic link, FIFO or other device) that the attacker has created in the
expected file location. Hence mkstemp() is
recommended, since it atomically creates the file. An attacker can guess the
file names produced by mktemp(). Whenever it is
possible, mkstemp() or
mkdtemp() should be used instead.

For this reason, ld(1) will output
a warning message whenever it links code that uses
mktemp().